At present, the main component of an active layer of a TFT in a liquid crystal display comprises silicon, such as, amorphous silicon or polysilicon. A TFT employing amorphous silicon as the active layer is hardly used in the situations requiring a large current and a rapid response, such as, an organic light emitting display, a display having a large size, a high resolution, and a high scanning frequency, or the like, due to its characteristic limitations (e.g., mobility, On-state current, etc.). A TFT employing polysilicon as the active layer can be used in an organic light emitting display due to its better characteristics than amorphous silicon; but the TFT has poor uniformity, so it is hard to be used for preparing a large size panel. Therefore, there is an increasing attraction to oxide semiconductor.
A TFT employing an oxide semiconductor as the active layer has better characteristics, such as, mobility, On-state current, switching characteristics, etc., than that employing amorphous silicon. Although certain characteristics of an oxide semiconductor are not as good as polysilicon, it still can satisfy the requirements of applications requiring a rapid response and a large current, such as, a display having a high frequency, a high resolution, and a large size, an organic light emitting display, etc. In general, an oxide has better uniformity than polysilicon, so it can be prepared by a sputtering method or the like without additional equipments, and thus costs can be low.
FIG. 1 is a structure view of an oxide-TFT in the prior art. In the process for fabricating an existing oxide-TFT, source/drain electrodes 105a, 105b are patterned by wet etching. Since an active layer 103 below the source/drain electrodes also can be etched by etchant, an etching blocking layer 104 is typically formed to protect the active layer 103 from being etched during etching of the source/drain metal layer.
The existing etch blocking layer is a single layer of SiOx. Since chemical bonds containing H, such as Si—H bond and N—H bond, may be generated during forming of SiOx, the etching blocking layer usually contains high concentration of H+. Further, the active layer is made of an oxide semiconductor, such as, IGZO, IZO, or the like, and the H+ ions inside both the etch blocking layer and an upper passivation layer may invade into the active layer, so oxygen holes may be changed in the active layer such as IGZO, the threshold voltage (Vth) may shift, and On/Off current (Ion/Ioff) may be reduced, etc. Thus, the property of the TFT is deteriorated, and the usage life of the TFT is shortened.